Non-dispersive infrared gas analyzer with interfering gas correction

ABSTRACT

An infrared gas analyzer for measuring low concentrations of a target gas, on the order of parts per million, in a sample gas is comprised of a gas sampling chamber, an infrared light source, and a power source for energizing the light source. A plurality of filters is provided to transmit infrared radiation at certain wavelengths. The wavelengths are chosen such that the effects of unwanted gases (such as water and carbon dioxide) can be removed from the final output signal. A plurality of infrared detectors are responsive to the filters for producing a plurality of electrical signals. A circuit is provided for combining the plurality of electrical signals to produce an output signal representative of the concentration of the target gas independently of other gases in the sample gas. A method of measuring low concentrations of a target gas is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable

FEDERALLY SPONSORED RESEARCH

Not applicable

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to non-dispersive infrared gas analyzers,and more particularly, to a non-dispersive infrared gas analyzer with acircuit for compensating for interfering gases that are present in asample gas, a circuit for stabilizing the temperature of infraredsensors, and/or a circuit for compensating for lamp degradation due tolamp aging effects.

Description of the Background of the Invention

Non-dispersive infrared (NDIR) gas analyzers are used to measure theconcentration of a particular target gas component of a sample gas. NDIRanalyzers measure the infrared radiation absorbed by the target gascomponent of the sample gas and thus give a measure of the concentrationof each component. NDIR analyzers typically have high measuringsensitivity and high selectivity. However, the sensitivity of NDIR gasanalyzers is dependent on the relative humidity of the operatingenvironment, the concentration of interfering gases such as carbondioxide, the thermal sensitivity of infrared detectors, and the effectsof lamp degradation on infrared light sources. Thus, NDIR gas analyzerstypically cannot measure the concentration of a target gas in the partsper million range.

FIG. 1 illustrates a conventional NDIR gas analyzer. The sample gas isintroduced either actively or passively into a sample gas measuringcell 1. Infrared rays are emitted into the measuring cell 1 from aninfrared source 2. An optical filter, such as a bandpass filter 3 havinga pass band wavelength equal to that of the target gas component islocated at one end of the measuring cell between the infrared source 2and a detection area 4. The infrared rays are absorbed in the measuringcell 1 according to the concentrations of the target gas components. Theunabsorbed rays are transmitted to the bandpass filter 3, whichtransmits the selected portion of the unabsorbed rays to an infrareddetector 5. The infrared detector 5 converts the sensed rays into anelectrical signal that represents the concentration of the target gascomponent which shows infrared absorption in the pass band wavelength ofthe bandpass filter 3.

The optical absorption of light due to the presence of the targetcomponent molecules in the measuring cell 1 is given by Beer's Law:

    I(λ)=I.sub.o (λ)e.sup.-ax                    ( 1)

a=k(λ)L

k=(λ) wavelength dependent gas absorption coefficient

L=optical path length

x=gas concentration

The absorption coefficients are dependent on wavelength and thus theappropriate absorption may be selected by choosing the optical filtercharacteristics carefully.

When there are three absorbing species in the measuring cell 1, equation(1) becomes:

    I(λ)=I.sub.o (λ)e.sup.-ax.sbsp.1 e.sup.-bx.sbsp.2 e.sup.-cx.sbsp.3                                          ( 2)

where now,

a, b, and c represent the absorption coefficients of three differentgases and x₁, x₂, and x₃ represent the corresponding concentrations ofthe three gases.

When the arguments of the exponentials are small, the followingapproximation of equation (2) may be used for three gases, for example:

    I(λ).tbd.I.sub.o (λ) 1-ax.sub.1 -bx.sub.2 -cx.sub.3 !(3)

The prior art includes variations on the NDIR gas analyzer as shown inFIG. 1. For example, U.S. Pat. No. 5,429,805, issued to Uno, et al.,discloses a Non-Dispersive Infrared Gas Analyzer Including Gas-FilledRadiation Source. The target gas component is enclosed in a sealedinfrared radiation source which has a transmission window that transmitsthe infrared rays into the measuring cell. Thus, the infrared rays inthe absorption center wavelength of the target gas component are removedin the radiation source.

Uno also discloses a motor with a rotating sector (chopper) which chopsthe infrared rays to interrupt the transmission of the rays into themeasuring cell. The light is chopped because infrared detectors such aspyroelectric detectors respond to intermittent radiation. A portion ofthe infrared rays in the measuring cell is absorbed by the gas to bemeasured and is distributed over the wavelength outside the centerabsorption band of the target gas component. The target gas componentshows a small absorption coefficient in the wavelength outside thecenter absorption band of the target gas.

Furthermore, Uno discloses an optical bandpass filter which is limitedto the pass band of the target gas. The optical bandpass filtertransmits a narrow wavelength range to the infrared detectors becausethe transmitted wavelength is the wavelength of the rays that were notabsorbed by the target gas minus the center absorption wavelength thatwas absorbed in the infrared source.

The prior art also includes variations of the composition and texture ofthe lining of the measuring cell. For example, Wadsworth, et al., U.S.Pat. No. 5,453,620, discloses an optically reflective surface on theinterior of the cell that has an irregular and constantly changingprofile.

There is a large concentration of humidity present in the atmosphere inmany environments in which NDIR gas analyzers are used. Water vapor mayhave an infrared absorption spectrum that overlaps that of the targetgas component. There is also a large concentration of carbon dioxidepresent in the atmosphere in many environments in which NDIR gasanalyzers are used. Carbon dioxide may have an infrared absorptionspectrum that overlaps that of the target gas component. The prior artdiscussed does not contain adequate means for compensating for suchhumidity or carbon dioxide dependencies of the disclosed NDIR gasanalyzers.

The prior art does not provide for optimization of the reflection ofinfrared rays in the sample gas measuring cell for improved illuminationof multiple detectors.

Infrared detectors such as those used in NDIR gas analyzers are heatdetectors, and thus the output of the analyzers are dependent on thetemperature of the infrared detectors. The analyzers are also sensitiveto the rate of temperature change. The prior art uses either active orpassive devices for controlling temperature variations in NDIR gasanalyzers. Passive devices include heat sinks or thermal insulation butthese devices cannot actively vary the temperature of the infrareddetectors. Active devices allow a higher degree of control of thethermal environment of the infrared detectors but require controlcircuitry that is customized for the gas analyzer. The prior art usessuch active devices as pulse width modulator temperature controllersthat can produce large temperature gradients.

During the useful life of an infrared radiation source, the lampdegrades over time. The degradation causes the intensity of the lightseen at the detector area of the NDIR gas analyzer to weaken over time.The prior art discussed does not disclose any method to compensate forsuch lamp aging effects.

Because the previously discussed problems greatly affect the sensitivityof NDIR gas analyzers, prior art NDIR gas analyzers are incapable ofmeasuring at sensitivities in the parts per million range. There is aneed for an NDIR gas analyzer that can measure the concentration of atarget gas component in a sample gas in the parts per million range.There is also a need for an NDIR gas analyzer that compensates for thehumidity present in the atmosphere of the device operating environment.Also, there is a need for an NDIR gas analyzer that can stabilize thetemperature of the infrared detectors so as to reduce the temperaturedependence of the device. Furthermore, there is a need for an NDIR gasanalyzer that can compensate for the effects of infrared lampdegradation. Also, there is a need for an NDIR gas analyzer that has asample gas cell with a reflective surface that maximizes opticalthroughout and uniformly illuminates the infrared detectors. There isalso a need for an NDIR gas analyzer that compensates for the carbondioxide present in the atmosphere of the device operating environment.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided anon-dispersive gas analyzer that produces an output signal whichrepresents the concentration of a target gas in a sample gas. A circuitfor maintaining the temperature of the infrared detectors of the gasanalyzer at a near constant temperature may also be provided. The gasanalyzer may also include a circuit for controlling the intensity of theinfrared light source such that the infrared light source emits infraredlight at a near constant intensity over time. A method for measuring lowconcentrations of a target gas in a sample gas is also provided.

An advantage of the present invention is to provide a non-dispersive gasanalyzer that compensates for the presence of interfering gases, such ashumidity and carbon dioxide, when the concentration of a target gas ismeasured in a sample gas. According to another embodiment, the presentinvention provides the advantage of compensating for the temperaturedependence of the analyzer by controlling the temperature of theinfrared detectors. According to another embodiment of the presentinvention, the advantage of compensating for infrared light sourcedegradation due to lamp aging effects is provided. Those advantages andbenefits, and others, will become apparent from the Description of thePreferred Embodiments hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, there is shown preferred embodiments ofthe invention wherein like reference numerals are employed to designatelike parts and wherein:

FIG. 1 is a view of a typical prior art non-dispersive infrared gasanalyzer;

FIG. 2A is a functional layout of a preferred embodiment of thenon-dispersive infrared gas analyzer of the present invention;

FIG. 2B is a magnified view of the striated inside walls of a sample gasmeasuring cell.

FIG. 3 is a simplified view of an active lamp control circuit of apreferred embodiment of the present invention;

FIG. 4 is a graph of the pass band for the target gas bandpass filter ofa preferred embodiment of the present invention;

FIG. 5 is a graph illustrating the absorption spectrum of a target gas,humidity and carbon dioxide;

FIG. 6 is a graph of the pass band for the reference bandpass filter ofa preferred embodiment of the present invention;

FIG. 7 is a graph of the pass band for the carbon dioxide bandpassfilter of a preferred embodiment of the present invention;

FIG. 8A is a partial view of an electrical circuit implementation of apreferred embodiment of the present invention; and

FIG. 8B is a partial view of an electrical circuit implementation of apreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures for the purpose of illustrating a presentpreferred embodiment of the invention only and not for the purpose oflimiting the same, the figures show a non-dispersive temperaturecompensated infrared gas analyzer having atmospheric humiditycompensation and atmospheric carbon dioxide compensation which isresistant to lamp aging.

More particularly and with reference to FIG. 2A, a functional layout ofa preferred embodiment of the present invention is shown. The NDIR gasanalyzer is generally depicted as 8. The analyzer includes a sample gasmeasuring cell 9, which has an infrared light source 10 located at oneend of the sample gas measuring cell 9 and is positioned such thatinfrared rays emitted by the infrared source 10 are emitted into thesample gas measuring cell 9. A parabolic reflector 11 reflects some ofthe infrared rays emitted by the infrared source 10 into the sample gasmeasuring cell 9.

The sample gas measuring cell 9 can be constructed of a reflectivematerial such as aluminum. The sample gas measuring cell 9 can be of anyshape that is practicable for reflecting infrared light, such asconical, cylindrical, or square. In a preferred embodiment of thepresent invention, the inside walls of the sample gas measuring cell 9are striated with longitudinal grooves 7, as seen in FIG. 2B, thatreflect and diffuse the infrared light along the direction of the pathof the light, while reducing backscatter of the rays. That achieves lowloss transmission of infrared light to an infrared detection mountingblock 12. The mounting block 12 is located at an end of the sample gasmeasuring cell 9 that is opposite the end where the infrared source 10is located. Such low loss transmission of infrared light due to thereduction of backscatter of rays has not been realized in the prior art,such as Wadsworth, et al., U.S. Pat. No. 5,453,620. The striationsformed on the inside of the sample gas measuring cell 9 can be createdby, e.g., passing abrasive materials through the cell duringmanufacture. Alternatively, flexible reflective tape on whichlongitudinal striations are formed may be used to line the sample gasmeasuring cell 9.

The infrared detection mounting block 12 is preferably comprised ofthree single bandpass filters: a carbon dioxide bandpass filter 13, areference bandpass filter 14, and a target gas and carbon dioxidebandpass filter 15. The infrared detection mounting block 12 preferablyfurther comprises three infrared detectors: a carbon dioxide infrareddetector 16, a reference infrared detector 17, and a target gas andcarbon dioxide infrared detector 18. The detectors 16, 17, and 18 arelocated adjacent to the bandpass filters 13, 14, and 15, respectively.The infrared detection mounting block 12 may be constructed of amaterial that possesses high thermal conductivity, such as aluminum. Theinfrared detection mounting block 12 can also be surrounded by athermally insulating compound, e.g. RTV silicone, to ensure thermalisolation of the infrared detector mounting block 12 from theenvironment.

One type of infrared detector that may be used in the detection block 12is a pyroelectric detector, which may be operated without cooling, isinexpensive, and has excellent sensitivity in detecting infraredradiation at certain wavelengths, e.g., in the 4 to 5 micron wavelengthrange. However, it can be understood by those skilled in the art thatmany types of infrared detectors may be used to achieve the sameresults, including semiconductor-based infrared detectors orthermal-type detectors.

Atmospheric gas enters and passes through the sample gas measuring cell9 through holes or perforations in the cell, and the target gascomponent of the sample gas absorbs the infrared rays at acharacteristic wavelength, carbon dioxide absorbs the infrared rays at acharacteristic wavelength, and water vapor absorbs the infrared rays ata characteristic wavelength. The unabsorbed rays at the abovecharacteristic wavelengths are passed by the respective bandpass filters13, 14, and 15 to the detectors 16, 17, and 18. The carbon dioxideinfrared detector 16 produces an electrical signal 19 that ischaracteristic of the concentration of carbon dioxide in the sample gas.The reference infrared detector 17 produces an electrical signal 20 thatis characteristic of the concentration of the infrared absorption of thewater vapor in the sample gas. The target gas and carbon dioxideinfrared detector 18 produces an electrical signal 21 that ischaracteristic of the concentration of the target gas and carbon dioxidein the gas sample. Electrical signals 19, 20, and 21 are conditioned andsignals 20 and 21 are input to an operational amplifier 22, whichsubtracts the reference signal 20 from the target gas and carbon dioxidesignal 21 to produce an intermediate signal 23, the voltage value ofwhich is not dependent on water vapor. The signals 19 and 20 are inputto an operational amplifier 24, which subtracts the reference signal 20from the carbon dioxide signal 19 to produce an intermediate signal 25.

The intermediate signal 25 is subtracted from the intermediate signal 23by an operational amplifier 26 to produce an output signal 27. Thevoltage value of the output signal 27 is thus proportional to theconcentration of the gas component to be measured and does not depend onthe concentration of carbon dioxide or water vapor in the sample gas.

Temperature changes at the detectors affect the voltage value of theoutput signal 27, which is given as:

    V.sub.0 30 a(T-T.sub.0)+b dT/dt                            (4)

where T is the temperature at the detectors obtained from a temperaturecontrol 41, T₀ is the temperature at the detector when the zeroadjustment was made at steady temperature, and V₀ is a part that is notaffected by temperature. Temperature sensor 29 senses the ambienttemperature of the gas analyzer 8. The residual temperature trim circuit28 adds residual temperature trim signal 30 that is given by

    V.sub.1 -a(T-T.sub.0)                                      (4a)

to the output signal 27. Thus, the residual temperature trim signal 30helps to eliminate the portion of the non-optical thermal signal whichis linear in the temperature.

An absolute humidity trim circuit 31 uses a voltage proportional to therelative humidity as sensed by a humidity sensor 32 to produce anabsolute humidity trim signal 33, the voltage of which is proportionalto the relative concentration of water vapor. The absolute humidity trimsignal 33 adjusts the output signal 27 to compensate for any residualwater sensitivity that was not removed after the two stages ofoperational amplifiers. This residual water sensitivity may not havebeen removed because of variations in bandpass filter characteristicsthat can be attributed to the manufacturing process of the filters.

Relative humidity is a measure of the amount of water vapor in theatmosphere relative to the maximum water vapor that the atmosphere cancarry at a given temperature. The equation that covers the temperaturevariation of the maximum water vapor is given by the Clausius-Clapeyronequation:

    P.sub.H.sbsb.2.spsb.O =P.sub.o l.sup.-L/RT                 (5)

P_(H20) is the partial pressure of water

P_(o) is a known constant

L is the latent heat of evaporation

R is the universal gas constant

T is the temperature

The concentration of water vapor at a relative humidity of R_(H) at agiven temperature T is given by: ##EQU1##

An evaluation of the temperature dependence of the concentration showsthat for a limited range of temperature near the operating temperatureof the sample gas cell, the temperature dependence may be linearizedsuch that: ##EQU2## β is the temperature coefficient β is approximately5% per degree Celsius under the conditions of operation in a preferredembodiment of the present invention and therefore a temperaturesensitive amplifier was added to the absolute humidity trim circuit 31to compensate for the temperature sensitivity β.

After the output signal 27 is adjusted by the residual temperature trimcircuit 28 and the absolute humidity trim circuit 31, it is input to anoutput display circuit 34. The output display circuit 34 may contain anLCD display 35, LED/buzzer alarms 36, or an analog voltage output 37that is buffered by an output buffer 38. Those skilled in the art willrecognize that other output devices may be connected to the outputsignal 27, such as a voltage monitor output or other combinations ofdigital or analog displays.

An active lamp control circuit 39 receives the reference signal 20 andcalculates the change in the reference signal 20 as a consequence oflight changes due to variations in the optical path, such as lightsource aging, sample gas cell degradation, or changes in absorption inthe passband of the reference detector. The active lamp control circuit39 ensures that a constant level of light is present at the referencedetector 17 by controlling a pulse driver 40 to adjust the power of eachpulse to the infrared source 10. The pulse driver 40 is preferably anoperational amplifier oscillator circuit that produces a square wavepulse at the approximate frequency of 1.5 Hz.

A simplified diagram of the lamp control circuit 39 is shown in FIG. 3.An operational amplifier 50 acts as an integrator and controls the lightoutput within a limited range to the pulse driver 40. The referencesignal 20, which is produced by the reference detector 17, feeds backthrough a reference feedback circuit 51 into the operational amplifier50, which provides a control signal to a lamp control component 52 ofthe lamp control circuit 39. The voltage value of the reference signal20 is indicated as V_(ref).

The response of the operational amplifier 50 may be given as: ##EQU3##

With feedback from the feedback circuit 51, the operational amplifiermay be characterized as: ##EQU4##

The output of the reference detector 17, reference signal 20 (V_(ref)),is held at the constant value V₊.

Because the reference detector 17 is chosen such that it has the samesensitivity to water vapor as the target gas and carbon dioxide detector18, a decrease in the reference signal 20 produces a correspondingincrease in light intensity and helps to compensate for water vaporabsorption seen by the target gas and carbon dioxide detector 18 whenwater vapor is present in the sample gas cell 9.

Returning now to FIG. 2A, a temperature control circuit 41 is an activecircuit which controls a detector heater 42 by raising or lowering theelectrical current supplied to the heater 42 proportional to deviationsin temperature of the detectors 16, 17, and 18. A thermistor 43 measuresthe temperature of the detectors 16, 17, and 18 and feeds themeasurement back to the temperature control circuit 41. The detectors16, 17, and 18, the infrared detector mounting block 12, the thermistor43, and the detector heater 42 comprise an optical detector head 44.

The equations governing the thermal response of the detectors 16, 17,and 18 can be written such that the signal S developed from a detectorcan be broken into two parts. One part is due to light falling on thedetector and the other part is due to the other temperature changes ofthe detector:

    S=S.sub.optical +S.sub.thermal                             (10)

S is the overall signal

S_(optical) is the signal due to light

S_(thermal) is the signal due to non-optical temperature changes

The gas detection capability of the detector is compromised if thethermal portion of the signal becomes comparable the optical portion.The thermal portion of the signal may be described as: ##EQU5## where Tis the temperature In the case of a single detector, it is desirable tohave the coefficients c₀ and c₁ as small as possible to minimize thethermal signal. When more than one detector is used, the coefficients c₀and c₁ may be matched to reduce or eliminate the thermal signal.##EQU6##

A description of an example of a non-dispersive gas analyzer designedaccording to the teachings of the present invention follows. The gasanalyzer was designed to measure the concentration of nitrous oxide (N₂O) in a sample gas. The target gas and carbon dioxide bandpass filter 15and the target gas detector 23 were designed to optimally pass anddetect infrared rays unabsorbed by nitrous oxide at its peaktransmission wavelength. FIG. 4 shows a preferred response of a nitrousoxide bandpass filter designed to pass the unabsorbed infrared rays at awavelength of nitrous oxide absorption.

The gas analyzer must be able to measure the concentration of nitrousoxide in the parts per million range and therefore must compensate forthe carbon dioxide and water vapor present in the sample gas. In thefollowing equations, nitrous oxide is represented by N, carbon dioxideby C, and water vapor by H. Thus, Beer's law from equation (3) may bewritten as:

    I.sup.N (λ)=I.sub.o.sup.N (λ)l.sup.-a.sbsp.N.sup.x.sbsp.N l.sup.-b.sbsp.N.sup.x.sbsp.C l.sup.-c.sbsp.N.sup.x.sbsp.H (16)

    a.sub.N =k.sub.N (λ)L                               (17)

    I.sup.C (λ)=I.sub.O.sup.C (λ)l.sup.-a.sbsp.C.sup.x.sbsp.N l.sup.-b.sbsp.x.sup.x.sbsp.C L.sup.-c.sbsp.C.sup.x.sub.N  (18)

    a.sub.C =k.sub.C (λ)L                               (19)

    I.sup.R (λ)=I.sub.o.sup.R (λ)l.sup.-a.sbsp.C.sup.x.sbsp.N l.sup.-b.sbsp.C.sup.x.sbsp.C l.sup.-C.sbsp.C.sup.x.sbsp.N (20)

    a.sub.R =k.sub.R (λ)L                               (21)

Equations (16)-(21) may be linearized as:

    I.sup.N (λ).tbd.I.sub.o.sup.N  1-a.sub.N x.sub.N -b.sub.N x.sub.C -c.sub.N x.sub.H !                                        (22)

    I.sup.R (λ).tbd.I.sub.o.sup.R  1-a.sub.R x.sub.N -b.sub.R x.sub.C -c.sub.R x.sub.H !                                        (23)

    I.sup.C (λ).tbd.I.sub.o.sup.C  1-a.sub.C x.sub.N -b.sub.C x.sub.C -c.sub.C x.sub.H !                                        (24)

Equations (22)-(24) may also be represented in vectormatrix form as:##EQU7##

FIG. 5 shows the percent transmission of infrared light for theidentified gas on the y-axis and the wavelength on the x-axis. Thelocation of the primary absorption band of nitrous oxide, shown as ashaded column in the figure, corresponds to some absorption by carbondioxide and water. Therefore, without compensation, the gas analyzerwould respond to carbon dioxide, water, and nitrous oxide. To subtractthe response due to water vapor from the outputted signal, the referencebandpass filter 14 must be designed to have a passband at which noabsorption by nitrous oxide occurs and absorption by water is matchedwith that at the selected passband shown in FIG. 4. FIG. 6 shows theresponse of a preferred reference filter. Also, to subtract the responsedue to carbon dioxide from the outputted signal, the carbon dioxidebandpass filter 13 must be designed to pass wavelengths associated withcarbon dioxide absorption, but does not transmit substantially atwavelengths associated with nitrous oxide absorption. FIG. 7 shows theresponse of a preferred carbon dioxide filter that is of one type thatis commonly commercially available.

The three detectors produce voltages that are proportional to the lightintensity signals received at each of the three detectors. Thesevoltages are given as:

    V.sup.N =V.sub.o.sup.N +α.sub.N  1-a.sub.N x.sub.N -b.sub.N x.sub.C -c.sub.N x.sub.H !                                        (26)

    V.sup.R =V.sub.o.sup.R +α.sub.R  1-a.sub.R x.sub.N -b.sub.R x.sub.C -c.sub.R x.sub.H !                                        (27)

    V.sup.C =V.sub.o.sup.C +α.sub.C  1-a.sub.C x.sub.N -b.sub.C x.sub.C -c.sub.C x.sub.H !                                        (28)

where:

α_(i) =voltage proportionality factors

From those voltages, signals are derived by subtraction of the referencevoltage (signal 20 in FIG. 1) from the N and C voltages (signals 21 and19, respectively, in FIG. 2). Those signals are given as: ##EQU8##

The absorption coefficients depend on the optical wavelengths selectedusing the appropriate bandpass filters. In the case of a nitrous oxidegas analyzer, the filter characteristics are given such that thefollowing relationships hold: ##EQU9## The water response and nitrousoxide sensitivity of the carbon dioxide bandpass filter are negligiblewith respect to the carbon dioxide sensitivity of the carbon dioxidebandpass filter, such that the following relationships hold:

    (c.sub.C -c.sub.R)x.sub.H <<(a.sub.C -a.sub.R)x.sub.N <<(b.sub.C -b.sub.R)x.sub.C                                          (34)

    i.e. (c.sub.C -c.sub.R)x.sub.H <<a.sub.C x.sub.N <<b.sub.C x.sub.C(35)

The signal equations from equations (29) and (30) thus can be simplifiedto:

    S.sub.N =S.sub.o.sup.N -a.sub.N x.sub.N -b.sub.N x.sub.C   (36)

    and S.sup.C =S.sub.o.sup.C -a.sub.C x.sub.N -b.sub.C x.sub.C(37)

This allows for the subtraction of the carbon dioxide signal to producea difference signal D^(N) which varies linearly only to theconcentration of N₂ O. D^(N) is thus given as: ##EQU10## D^(N) thenrepresents a signal which varies linearly to the N₂ O concentrationX_(N) in the presence of water vapor and carbon dioxide. In ourinstrument, since b_(N) /b_(C) <<1, the sensitivity of the nitrousdifference signal D^(N) of equation (38) is controlled by the nitrousabsorption of the nitrous channel a_(N).

FIGS. 8A and 8B are views of circuit layouts of an implementation of apreferred embodiment of a non-dispersive gas analyzer that measures theconcentration of nitrous oxide in a sample gas. The analyzer wasconstructed according to the principles embodied in FIGS. 2-7 and theaccompanying explanations. FIG. 8A shows a main signal channel circuit60. The main signal channel circuit 60 performs amplification anddifferencing of the outputs of the three infrared detectors. Anamplification circuit 61 of the main signal channel circuit 60 convertsthe optical signal produced by each detector into a level electricalsignal. A reference subtraction circuit 62 of the main signal channelcircuit 60 subtracts the signal representing the concentration of watervapor present in the sample gas, the light intensity in the passband ofthe reference detector, and any noise that is present from the carbondioxide detector signal and the target gas and carbon dioxide signal. Acarbon dioxide subtraction circuit 63 of the main signal channel circuit60 subtracts the signal representing the concentration of carbon dioxidepresent in the sample gas from the target gas and carbon dioxide signal.

The main signal channel circuit 60 also has a zero calibration circuit64 which is used to adjust the gas analyzer to its zero setting.Calibration to the zero setting is accomplished by adjusting apotentiometer in the zero calibration circuit 64 until the analyzerdisplay outputs a "0" reading in an environment that has no nitrousoxide present.

The main signal channel circuit 60 also has a span calibration circuit65 which is used to calibrate the span of the analyzer in the presenceof a known concentration of nitrous oxide. The potentiometer of the spancalibration circuit 65 is adjusted when the sample gas cell of theanalyzer is filled with a known concentration of nitrous oxide.

FIG. 8A shows a heater control circuit 66 which is used to maintain aconstant temperature of the optical detector head. This allows for themeasurement of small optical signals at the detectors because thecircuit minimizes the signal associated with the thermal sensitivity ofthe detectors. In the circuit configuration shown as used on theanalyzer, the heater control circuit 66 can hold the temperature of theoptical detector head within 0.01 degree Celsius of the desiredtemperature.

An active lamp control circuit 67 compensates for the degradation of theinfrared light source by adjusting the intensity of the signal producedby the lamp driver.

A relative humidity trim circuit 70 is shown in FIG. 8B. The relativehumidity trim circuit 70 compensates for variations in humidity responsedue to filter-to-filter variations. A small signal is added orsubtracted to the output signal by the relative humidity trim circuit 70to trim the humidity response.

An output display circuit 71 displays the outputted value of the nitrousoxide concentration of the sample gas in various forms. The outputdisplay circuit 71 in the embodiment shown uses LCD and LED displays anda buzzer.

A voltage output circuit 72 provides a 0-5 volt DC analog signal that issuitable for connection to a data logger or a centralized multipointcontrol/monitoring system. A current output circuit 73 provides a 4-20mA output signal that can be used for data logging purposes.

The present invention provides solutions to the problem ofnon-dispersive infrared gas analyzers that are in use today that cannotmeasure the concentrations of a target gas in the parts per millionrange because of humidity and carbon dioxide concentrations present in asample gas, light source degradation over time, and/or the thermaldependencies of optical detectors. It will be understood, however, thatvarious changes in the details, materials, and arrangements of partswhich have been herein described and illustrated to explain the natureof the invention may be made by those skilled in the art within theprinciple and scope of the invention as expressed in the appendedclaims.

What is claimed is:
 1. An infrared gas analyzer for measuring low concentrations of nitrous oxide in a sample gas, comprising:a gas sampling chamber having a first end and a second end; an infrared light source located at said first end of said gas sampling chamber such that light produced by said infrared light source projects into said gas sampling chamber; a power source for energizing said infrared light source; a plurality of filters located at said second end of said gas sampling chamber, wherein one of said filters is a first bandpass filter for transmitting wavelengths of infrared light at an optimal wavelength for nitrous oxide and at a wavelength associated with carbon dioxide in the sample gas and wherein one of said filters is a second bandpass filter for transmitting wavelengths of infrared light at a wavelength associated with water; a plurality of infrared detectors each responsive to one of said filters for producing a plurality of electrical signals representative of the amount of detected infrared light; and a circuit for combining said electrical signals to produce an output signal representative of the concentration of nitrous oxide independently of other gases in the sample gas.
 2. The infrared gas analyzer of claim 1 further comprising means for controlling the intensity of said infrared light source, said means being responsive to one of said infrared detectors for producing a signal input to said power source such that said infrared light source emits infrared light at a substantially constant intensity over time.
 3. The infrared gas analyzer of claim 1 wherein said power source produces pulses for energizing said infrared light source.
 4. The infrared gas analyzer of claim 1 further comprising:a temperature sensor responsive to the temperature of said infrared detectors; and means, responsive to said temperature sensor, for maintaining the temperature of said infrared detectors at a substantially constant value.
 5. The infrared gas analyzer of claim 1 wherein said gas sampling chamber has a striated lining.
 6. The infrared gas analyzer of claim 1 wherein said gas sampling chamber has a striated interior surface.
 7. The infrared gas analyzer of claim 1 further comprising a second temperature sensor and a residual temperature trim circuit responsive to said second temperature sensor for modifying said output signal to compensate for large temperature variations of said infrared gas analyzer.
 8. The gas analyzer of claim 1 further comprising a relative humidity sensor and an absolute humidity circuit responsive to said humidity sensor for modifying said output signal to compensate for any residual water sensitivity of said gas analyzer.
 9. The infrared gas analyzer of claim 1 wherein one of said filters passes infrared light at a wavelength of approximately 4.5 microns, one of said filters passes infrared light at a wavelength of approximately 4 microns, and one of said filters passes infrared light at a wavelength of approximately 4.3 microns.
 10. An infrared gas analyzer for measuring low concentrations of a target gas in a sample gas, comprising:a gas sampling chamber having a first ends a second end and a striated lining; an infrared light source located at said first end of said gas sampling chamber such that light produced by said infrared light source projects into said gas sampling chamber; a power source for energizing said light source; a filter located at said second end of said gas sampling chamber and responsive to a wavelength of absorption for the target gas; an infrared detector responsive to said filter for producing an electrical signal representative of the concentration of the target gas; and means for controlling the intensity of said infrared light source, said means being responsive to said infrared detector for producing a signal input to said power source such that said infrared light source emits infrared light at a substantially constant intensity over time.
 11. The infrared gas analyzer of claim 10 wherein said filter is a bandpass filter.
 12. The infrared gas analyzer of claim 10 further comprising:a temperature sensor responsive to the temperature of said infrared detector; and means, responsive to said temperature sensor, for maintaining the temperature of said infrared detector at a substantially constant value.
 13. The infrared gas analyzer of claim 10 further comprising a second temperature sensor and a residual temperature trim circuit responsive to said second temperature sensor for modifying said output signal to compensate for large temperature variations of said infrared gas analyzer.
 14. The infrared gas analyzer of claim 10 further comprising a relative humidity sensor and an absolute humidity circuit responsive to said humidity sensor for modifying said output signal to compensate for any residual water sensitivity of said gas analyzer.
 15. An infrared gas analyzer for measuring low concentrations of a target gas in a sample gas, comprising:a gas sampling chamber having a first end, a second end, and a striated lining; means for producing infrared light and for projecting the infrared light into said first end of said gas sampling chamber; a plurality of filters located at said second end of said gas sampling chamber; a plurality of infrared detectors each responsive to one of said filters for producing a plurality of electrical signals representative of the amount of detected infrared light; means for combining said electrical signals to produce an output signal representative of the concentration of the target gas; a temperature sensor responsive to the temperature of said infrared detectors; and means, responsive to said temperature sensor, for maintaining the temperature of said infrared detectors at a substantially constant value.
 16. The infrared gas analyzer of claim 15 wherein one of said filters is a bandpass filter which has a passband wavelength of approximately 4.3 microns.
 17. The infrared gas analyzer of claim 16 wherein another of said filters is a bandpass filter which has a passband wavelength of approximately 4 microns.
 18. The infrared gas analyzer of claim 15 wherein said means for producing infrared light includes an infrared light source and a power source producing pulses for energizing said light source.
 19. The infrared gas analyzer of claim 15 further comprising a second temperature sensor and a residual temperature trim circuit responsive to said second temperature sensor for modifying said output signal to compensate for large temperature variations of said infrared gas analyzer.
 20. The infrared gas analyzer of claim 15 further comprising a humidity sensor and a humidity circuit responsive to said humidity sensor for modifying said output signal to compensate for any residual water sensitivity of said gas analyzer.
 21. The infrared gas analyzer of claim 15 wherein one of said filters passes infrared light at a wavelength of approximately 4.5 microns, one of said filters passes infrared light at a wavelength of approximately 4 microns, and one of said filters passes infrared light at a wavelength of approximately 4.3 microns.
 22. An infrared gas analyzer for measuring the concentration of nitrous oxide in a sample gas, comprising:a gas sampling chamber having a first end, a second end, and a lining that is longitudinally striated; an infrared light source located at said first end of said gas sampling chamber such that light produced by said infrared light source projects into said gas sampling chamber; a power source for energizing said infrared light source; a first bandpass filter, a second bandpass filter, and a third bandpass filter, said bandpass filters located at said second end of said gas sampling chamber; a first infrared detector responsive to said first bandpass filter for producing a first electrical signal representative of the amount of detected infrared light at a wavelength corresponding to an absorption wavelength of water; a second infrared detector responsive to said second bandpass filter for producing a second electrical signal representative of the amount of detected infrared light at a wavelength corresponding to an absorption wavelength of carbon dioxide; a third infrared detector responsive to said third bandpass filter for producing a third electrical signal representative of the amount of detected infrared light at a wavelength corresponding to an absorption wavelength of nitrous oxide; an electrical circuit for combining said first electrical signal, said second electrical signal, and said third electrical signal to produce an output signal representative of the concentration of the nitrous oxide independently of the carbon dioxide and water in the sample gas; a temperature sensor responsive to the temperature of said infrared detectors; a circuit, responsive to said temperature sensor, for maintaining the temperature of said infrared detectors at a substantially constant value; and a control circuit for controlling the intensity of said infrared light source, said control circuit being responsive to one of said infrared detectors for producing a signal input to said power source such that said infrared light source emits infrared light at a substantially constant intensity over time.
 23. The infrared gas analyzer of claim 22 wherein said first bandpass filter has a passband wavelength of approximately 4 microns.
 24. The infrared gas analyzer of claim 23 wherein said second bandpass filter has a passband wavelength of approximately 4.3 microns.
 25. The infrared gas analyzer of claim 24 wherein said third bandpass filter has a passband wavelength of approximately 4.5 microns.
 26. The infrared gas analyzer of claim 25 wherein said infrared detectors are pyroelectric detectors.
 27. The infrared gas analyzer of claim 22 wherein said gas sampling chamber is constructed of a material selected from the group consisting of aluminum and gold.
 28. The infrared gas analyzer of claim 22 further comprising a second temperature sensor and a residual temperature trim circuit responsive to said second temperature sensor for modifying said output signal to compensate for large temperature variations of said infrared gas analyzer.
 29. The infrared gas analyzer of claim 28 further comprising a relative humidity sensor and an absolute humidity circuit responsive to said humidity sensor for modifying said output signal to compensate for any residual water sensitivity of said gas analyzer.
 30. The infrared gas analyzer of claim 22 wherein said analyzer measures concentrations of nitrous oxide at levels of parts per million.
 31. An infrared gas analyzer for measuring low concentrations of a target gas in a sample gas, comprising:a gas sampling chamber having a first end, a second end, and a striated interior surface; an infrared light source located at said first end of said gas sampling chamber such that light produced by said infrared light source projects into said gas sampling chamber; a power source for energizing said light source; a filter located at said second end of said gas sampling chamber and responsive to a wavelength of absorption for the target gas; an infrared detector responsive to said filter for producing an electrical signal representative of the concentration of the target gas; and means for controlling the intensity of said infrared light source, said means being responsive to said infrared detector for producing a signal input to said power source such that said infrared light source emits infrared light at a substantially constant intensity over time.
 32. The infrared gas analyzer of claim 31 wherein said filter is a bandpass filter.
 33. The infrared gas analyzer of claim 31 further comprising:a temperature sensor responsive to the temperature of said infrared detector; and means, responsive to said temperature sensor, for maintaining the temperature of said infrared detector at a substantially constant value.
 34. The infrared gas analyzer of claim 31 further comprising a second temperature sensor and a residual temperature trim circuit responsive to said second temperature sensor for modifying said output signal to compensate for large temperature variations of said infrared gas analyzer.
 35. The infrared gas analyzer of claim 31 further comprising a relative humidity sensor and an absolute humidity circuit responsive to said humidity sensor for modifying said output signal to compensate for any residual water sensitivity of said gas analyzer.
 36. An infrared gas analyzer for measuring low concentrations of a target gas in a sample gas, comprising:a gas sampling chamber having a first end, a second end, and a striated interior surface; means for producing infrared light and for projecting the infrared light into said first end of said gas sampling chamber; a plurality of filters located at said second end of said gas sampling chamber; a plurality of infrared detectors each responsive to one of said filters for producing a plurality of electrical signals representative of the amount of detected infrared light; means for combining said electrical signals to produce an output signal representative of the concentration of the target gas; a temperature sensor responsive to the temperature of said infrared detectors; and means, responsive to said temperature sensor, for maintaining the temperature of said infrared detectors at a substantially constant value.
 37. The infrared gas analyzer of claim 36 wherein one of said filters is a bandpass filter which has a passband wavelength of approximately 4.3 microns.
 38. The infrared gas analyzer of claim 37 wherein another of said filters is a bandpass filter which has a passband wavelength of approximately 4 microns.
 39. The infrared gas analyzer of claim 36 wherein said means for producing infrared light includes an infrared light source and a power source producing pulses for energizing said light source.
 40. The infrared gas analyzer of claim 36 further comprising a second temperature sensor and a residual temperature trim circuit responsive to said second temperature sensor for modifying said output signal to compensate for large temperature variations of said infrared gas analyzer.
 41. The infrared gas analyzer of claim 36 further comprising a humidity sensor and a humidity circuit responsive to said humidity sensor for modifying said output signal to compensate for any residual water sensitivity of said gas analyzer.
 42. The infrared gas analyzer of claim 36 wherein one of said filters passes infrared light at a wavelength of approximately 4.5 microns, one of said filters passes infrared light at a wavelength of approximately 4 microns, and one of said filters passes infrared light at a wavelength of approximately 4.3 microns.
 43. An infrared gas analyzer for measuring the concentration of nitrous oxide in a sample gas, comprising:a gas sampling chamber having a first end, a second end, and a striated interior surface; an infrared light source located at said first end of said gas sampling chamber such that light produced by said infrared light source projects into said gas sampling chamber; a power source for energizing said infrared light source; a first bandpass filter, a second bandpass filter, and a third bandpass filter, said bandpass filters located at said second end of said gas sampling chamber; a first infrared detector responsive to said first bandpass filter for producing a first electrical signal representative of the amount of detected infrared light at a wavelength corresponding to an absorption wavelength of water; a second infrared detector responsive to said second bandpass filter for producing a second electrical signal representative of the amount of detected infrared light at a wavelength corresponding to an absorption wavelength of carbon dioxide; a third infrared detector responsive to said third bandpass filter for producing a third electrical signal representative of the amount of detected infrared light at a wavelength corresponding to an absorption wavelength of nitrous oxide; an electrical circuit for combining said first electrical signal, said second electrical signal, and said third electrical signal to produce an output signal representative of the concentration of the nitrous oxide independently of the carbon dioxide and water in the sample gas; a temperature sensor responsive to the temperature of said infrared detectors; a circuit, responsive to said temperature sensor, for maintaining the temperature of said infrared detectors at a substantially constant value; and a control circuit for controlling the intensity of said infrared light source, said control circuit being responsive to one of said infrared detectors for producing a signal input to said power source such that said infrared light source emits infrared light at a substantially constant intensity over time.
 44. The infrared gas analyzer of claim 43 wherein said first bandpass filter has a passband wavelength of approximately 4 microns.
 45. The infrared gas analyzer of claim 44 wherein said second bandpass filter has a passband wavelength of approximately 4.3 microns.
 46. The infrared gas analyzer of claim 45 wherein said third bandpass filter has a passband wavelength of approximately 4.5 microns.
 47. The infrared gas analyzer of claim 46 wherein said infrared detectors are pyroelectric detectors.
 48. The infrared gas analyzer of claim 43 wherein said gas sampling chamber is constructed of a material selected from the group consisting of aluminum and gold.
 49. The infrared gas analyzer of claim 43 further comprising a second temperature sensor and a residual temperature trim circuit responsive to said second temperature sensor for modifying said output signal to compensate for large temperature variations of said infrared gas analyzer.
 50. The infrared gas analyzer of claim 49 further comprising a relative humidity sensor and an absolute humidity circuit responsive to said humidity sensor for modifying said output signal to compensate for any residual water sensitivity of said gas analyzer.
 51. The infrared gas analyzer of claim 43 wherein said analyzer measures concentrations of nitrous oxide at levels of parts per million.
 52. An infrared gas analyzer for measuring low concentrations of a target gas in a sample gas, comprising:a gas sampling chamber having a first end, a second end, and a striated lining; an infrared light source located at said first end of said gas sampling chamber such that light produced by said infrared light source projects into said gas sampling chamber; a power source for energizing said infrared light source; a plurality of filters located at said second end of said gas sampling chamber, wherein one of said filters is a first bandpass filter for transmitting wavelengths of infrared light at an optimal wavelength for the target gas and at a wavelength associated with a first other gas in the sample gas; a plurality of infrared detectors each responsive to one of said filters for producing a plurality of electrical signals representative of the amount of detected infrared light; and a circuit for combining said electrical signals to produce an output signal representative of the concentration of the target gas independently of other gases in the sample gas.
 53. An infrared gas analyzer for measuring low concentrations of a target gas in a sample gas, comprising:a gas sampling chamber having a first ends a second end, and a striated interior surface; an infrared light source located at said first end of said gas sampling chamber such that light produced by said infrared light source projects into said gas sampling chamber; a power source for energizing said infrared light source; a plurality of filters located at said second end of said gas sampling chamber, wherein one of said filters is a first bandpass filter for transmitting wavelengths of infrared light at an optimal wavelength for the target gas and at a wavelength associated with a first other gas in the sample gas; a plurality of infrared detectors each responsive to one of said filters for producing a plurality of electrical signals representative of the amount of detected infrared light; and a circuit for combining said electrical signals to produce an output signal representative of the concentration of the target gas independently of other gases in the sample gas. 